Production of isoprene



United States Patent Ofiice 3,233,006 Patented Feb. 1, 1966 3,233,006 PRODUCTION F ESQPFENE V Emmett H. Burk, .ln, Hazel Crest, Byron W. Turnquest, Chicago, and William D. Hofiman, Park Forest, 1111., assignors to Sinclair Research, inn, Wilmington, Del., a corporation of Delaware No Drawing. Filed Nov. 1, 1961, der. No. 149,159 6 Claims. (Cl. 260-430) The present invention relates to the production of iso- ,prene, more particularly to the production of isoprene by the pyrolysis of a halogenated hydrocarbon.

In view of the similarity of cis-polyisoprene, i.e. synthetic natural rubber, to natural rubber, the demand for isoprene is increasing. The availability. and cost of isoprene, however, present formidable barriers to the commercial production of synthetic natural rubber. Thus, there is a continuing search for new sources of isoprene production.

A great deal of research has been directed recently to the production of isoprene by dehydrogenation of C stocks by processes similar to these employed for the prodnotion of butadienes. However, these processes have disadvantages which limit their usefulness. Dehydrogenation of isopentane, which is available in relatively large quantities from normal refinery streams and natural gas or field gas streams, is ordinarily carried out by a one-step process such as a process using a chromiumalumina catalyst. In this process, isomerization occurs during dehydrogenation resulting in the formation of one pound of piperylen (straight chain C diolefins) for every 2.5 pounds of isoprene in the product.

Furthermore, the quantity of isoamylenes which are available from catalytic cracking or steam cracking of gas oil is quite limited relative to the potential demand for .isoprene according to the predicted potential demand for isoprene according to the predicted shortage of 600,000 long tons of natural rubber by 1965. Moreover, none of the special catalytic methods is capable of effecting dehydrogenation to isoprene in a desirably selective manner. In all cases, an appreciable amount of the hydrocarbon feed and/or reaction products is consumed in side reactions such as cracking, polymerization and oxidation.

It has also been reported that the thermal cracking of olefins such as Z-methylpentene-Z atfords a relatively simple method for isoprene production. In obtaining maximum selective yields under this process, however, it has been found that the degree of cracking is relatively low. Low conversion levels are disadvantageous in that they complicate the separation of the isoprene from the iodine, results in surprisingly high selective yields of 6O isoprene at high conversion levels with large concentr ations of isoprene in the C reaction product. Obtaining large concentrations of isoprene in the C reaction product is particularly important from the viewpoint of isoprene recovery. Due to the close boiling points of the C components, isoprene must often be concentrated by such expensive procedures as extractive distillation or solvent extraction.

The 3,3-dimethyl alkyl halide tteedstock of the present invention has the formula:

CH; RCH2CH2X CH: wherein R is an alkyl radical of 1 to 6 carbon atoms, straight or branch chained and with the higher number carbon atom radicals (i.e. of 3 to 6 carbons), the branched structure being preferred; and X is a halogen such as chlorine, bromine, iodine and fluorine. The feedrnaterials of the present invention are readily available, being produced by several methods well known in the art.

1-chloro-3,3-dirnethylbutane, for example, can be prepared as described by Louis Sclrrnerling in the Journal of the American Chemical Society, 67, 1152-54 (1945). Briefly, the process involves reacting 1 111101 of ethylene with t-butyl chloride using a Friedel-Crafts catalyst such as AlCl FeCl BiCl or ZnCl The reaction can take place at atmospheric pressure when employing the reactive Friedel-Crafts catalyst such as AlCl advantageously at a temperature of about C. Under the conditions listed by Schrnerling, i.e. complete conversion, the yield of l-chloro-3,3-dimethylbutane was reported as 75% theory. The other halides can be made by analogous procedures. Suitable alkyl halide feeds for use in the present invention are, for instance, 1-halo-3,3-dimethylbutane; 1-halo-3,3-dimethylpentane; 1-halo-3,3-dimethylhexane; 1-halo-3,3,4-trimethylpentane, l-halo 3,3- dirnethyl-5,S-dimethylhexane; etc. The alkyl halides can be susbtituted with non-intenferrin g groups if desired.

The hydrogen halide catalyst of the present invention can be hydrogen iodide or hydrogen brornide per se or the catalyst can be supplied by the iodide or bromide compounds which decompose and/ or dissociate under the reaction conditions to produce the hydrogen iodide or bromide and whose presence is not other-wise detrimental to the desired reaction. Illustrative of suitable halides are hydrocarbon iodides or bromides such as methyl iodide or bromide, tertiary butyl iodide or bromide, elemental iodine or bromine, or inorganic iodides and bromides such as hydrogen iodide or bromide. The hydrogen bromide or hydrogen iodide can b supplied by the feedstock, e.g. 3,B-dimethylalkyliod-ide and 3,3-dimethylalltylbrornide. The cracking of these feeds alone has been ttound suflicient to provide the hydrogen bromide or hy drogen iodide catalyst under the reaction conditions and gives highly selective yields of isoprene without added hydrogen halide catalyst.

Th amount of hydrogen iodide or hydrogen bromide present during the cracking reaction is sufficient to give a catalytic effect and is usually at least about 1 rnol percent, preferably about 2-10 mole percent. There does not appear to be any advantage in employing over say about mol percent of the iodide or bromide based on the alkyl halide feed.

In accordance with the present invention, the 3,3-dirnethylalkyl halide feed is subjected to pyrolysis at a minimum temperature of about 1150 F. or 1200 F. up to about 1600 F. and in the presence of hydrogen iodide or hydrogen bromide under vacuum or in the presence of an inert gas diluent such as helium, steam, etc. Op-

crating conditions can be adjusted to give an alkyl halide partial pressure of up to about 0.5 atmospheres, for instance, about 0.05 to 0.5 atmospheres, and a contact time suificient to produce isoprene, for instance, about .001 to 1 second. The inert gas, if employed can be supplied in a molar ratio out at least about 1 rnol, for instance, about 140 or 100 or more mols, preferably about 15-30 mols per mol of alkyl halide feed. Under preferred conditions, the alkyl halide partial pressure is about 0.1 atmosphere and the contact time is about 0.05 to 0.1 second. A reaction temperature of about 1200 to 1500 F. is preferred.

If desired, the inert gas can be employed as the heating medium to bring the feedstock rapidly to the cracking temperatures. This can conveniently be done by heating the inert gas to a temperature abov that desired for conducting the cracking operation, generally at least 77 F. higher than the reaction temperature and not above say 390 F. of the reaction temperature. The inert gas is then quickly mingled with the alkyl halide which is at a temperature below that at which any reactions occur. The temperature to which the inert gas is to be heated can be readily determined from the specific heat of the gas, the molar ratios involved, etc. The product from the thermocracking process is quenched to a temperature of about 500 F. or below and fractionated in ordinary fractionating equipment to obtain isoprene of at least about 95% purity.

The following examples will serve to illustrate the present invention.

EXAMPLE I A 12-inch furnace was equipped with a quartz reactor. Ceramic packing was used as a heat transfer medium. 1-chloro-3,3-dimethylbutane containing rnol percent of hydrogen iodide was introduced into a stream of steam and the mixture was passed through the reaction zone under the conditions shown in Table I below. After leaving the reaction zone, the gas stream was analyzed. For comparison, a run was also made without the added hydrogen iodide. The results are shown in Table I:

Table I Catalyst None 5 mol percent HI Temp, F 1, 260 1, 290 Conversion 99 99 M01. Steam/M01. Feed /1 10/1 Products (0 Wt. Percent (Halogen ree Isobutylene 22. 2 14. 1 Other C4 6. 3 4. 4 3-methy1butene-1. 0. 5 2. 2 2-rnethylbutene-l 2. 4 4. 2 2-methylbuteue-2 5. 8 S. 9 Isoprene 32. 1 47. 8 3.3-dimethylbutene-1 2t). 2 1t). 9 Vinyl Chloride 6. 0 1. 8 Others 3. 4 5. 5

Examination of the data of Table I shows that conducting the pyrolysis of the 1-ch1oro-3,3-dimethylb'utane in the presence of hydrogen iodide improves the yield of isoprene at high conversion levels and provides large concentrations of isoprene in the C reaction product.

EXAMPLE II 1+bromo-3,3-dimethylpentane containing 5 mol percent of hydrogen iodide was thermally cracked in accordance with the general method of Example I and under the condilliQIlS shown in Table 11 below. For comparison, a run.

was made without hydrogen iodide addition. The results are shown in Table 11 below:

The data of Table II demonstrates that although there is improvement when the hydrogen iodide catalyst is added, the cracking of the alkyl bromide alone is sufficient to give high selective yields and large concentrations of isoprene in the C fraction at high conversion levels.

We claim:

11. A process for producing isoprene which consists essentially of subjecting to pyrolysis in the vapor phase a halogenated hydrocarbon having the structural formula:

wherein R is an alkyl radical of 16 carbon atoms, at a temperature of about 11501600 P1, and a halogenated hydrocarbon partial pressure of up to about 0.5 atmospheres and in the presence of hydrogen halide in an amount of about 1 to 20 mol percent and sulficient to give a catalytic effect, the halogen atom of the hydrogen halide having an atomic number of -53.

2. The process of claim 1 wherein the hydrogen halide is hydrogen iodide.

3. The process of claim 1 wherein the halogenated hydrocarbon is 1 chloro-3,3-dimethylbutane.

4. The process of claim 3 wherein the hydrogen halide is hydrogen iodide.

5. A process for producing isoprene which consists essentially of subjecting to pyrolysis in the vapor phase a halogenated hydrocarbon having the structural formula:

wherein R is an alkyl radical or 16 carbon atoms, at a temperature of about 1150-1600 F., and a halogenated hydrocarbon partial pressure of up to about 0.5 atmospheres and in the presence of hydrogen iodide in an amount of about 1 to 20 mol percent and sufficient to give a catalytic efiect.

ii. The process of claim 5 wherein the halogenated hydrocarbon is 1- brorno-3,3-dimethylbutane.

References Cited by the Examiner UNITED STATES PATENTS 2,288,580 6/1942. Baehr z 260680 2,314,335 3/ 1943 Frey 260-677 FOREIGN PATENTS 1,251,127 12/1960 France.

PAUL M. COUGHLAN, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner. 

1. A PROCESS FOR PRODUCING ISOPRENE WHICH CONSISTS ESSENTIALLY OF SUBJECTING TO PYROLYSIS IN THE VAPOR PHASE A HALOGENATED HYDROCARBON HAVING THE STRUCTURAL FORMULA: 